This invention relates to a process for continuously rolling seamless metal pipes.
There are two types of continuous seamless metal (such as steel) pipe rolling mills; the full-floating mandrel mill and the semi-floating mandrel mill. These continuous rolling mills change the outside diameter and wall thickness of a tubular blank, using a series of substantially circular passes formed by driven paired rolls and a mandrel, having a length that is greater than the distance between the first and last roll stands and the length of the tubular blank, adapted to be passed through the workpiece. The difference betwen the two types lies in how the mandrel is restrained during rolling. On the full-floating mill, no other force than the rolling force acts on the mandrel during rolling. With the semi-floating mill, the mandrel moves forward at a constant speed, restrained by a thrust block. As a consequence, the mandrel of the full-floating mill leaves the rolling mill with the rolled pipe to a stripper on the exit side thereof, where they are separated. By contrast, the semi-floating mill has an extractor, comprising three to four pairs of rolls, on its exit side to take the pipe off the mandrel. On completion of rolling, therefore, the mandrel remains inside the continuous rolling mill, with the rear end thereof held by the thrust block. As soon as the pipe and mandrel clear, the full-floating mill can start the next rolling, allowing a very high production rate per unit time. On completion of rolling, the semi-floating mill must retract the thrust block to return the mandrel onto an entry table. The next rolling cannot be started until the mandrel has been inserted in the next tubular blanks and moved forward to the starting position and the blank sent into the rolling mill by feed rolls. The result is a much lower production rate per unit time.
On the full-floating mill, just the same, the mandrel speed increases sharply every time the front or tail end of the workpiece passes between the adjacent rolls. This breaks the balance of volume velocity at each roll, causing a sharp change in the deformation process of the workpiece. This change results in localized extraordinary deformation, entailing dimensional variations, both longitudinally and circumferentially. To prevent overfills that might result from such sharp dimensional variations, the conventional full-floating mill has had to provide a large recess at the edges of the roll groove. To facilitate the withdrawing of the mandrel from the rolled pipe, in addition, their diameters must be differentiated by several millimeters to provide adequate clearance therebetween. As a consequence, four longitudinal projections, known as ridges, form on the internal surface of the pipe, damaging its dimensional accuracy. The amount of reduction per pass has been limited too, calling for extra pipe deforming steps and extra energy consumption. Common practice is to drive each roll set with an independent DC motor. Because of a large torque needed for biting the workpiece, motors, with very large capacities, which are needed only temporarily, are used. For steady rolling, especially the motors for the first and second roll stands require only one-third or less of the torque needed for biting. Evidently, costly high-capacity motors are not in full-time service.
For increasing the dimensional accuracy of the rolled pipe, proposal has been made to control the peripheral speed of the rolls with the progress of the workpiece. But a capacity increase in the roll drive system, comprising a motor, reduction gear and spindle, increases moment of inertia, weakening the response to the control.
On the semi-floating mill, the movement of the mandrel is kept at a constant speed that is lower than the travel speed of the workpiece at any part of the mill. By this means, friction between the mandrel and the internal surface of the workpiece is oriented in the same direction at all roll stands, so that a constant volume velocity can be maintained during rolling. This permits roll passes to be designed closer to true circle, which is conductive to the improvement of the dimensional accuracy of the rolled pipe. To produce this condition, the speed of the mandrel must be kept at a rate lower than the peripheral speed at the bottom of the first roll set throughout the entire rolling process. This, however, calls for still greater biting torques, and still greater motor powers, than on the full-floating mill. Besides, irregular biting is likely to occur. This sets limits on the reduction of the diameter, and the arc of contact of rolls. Consequently, the time during which each unit length of the mandrel is in contact with the workpiece increases to shorten its service life.
Continuously pulled by the thrust block during rolling, the mandrel of the semi-floating mill is subjected to a large axial tensile stress, which accelerates the development of cracks on the mandrel surface and shortens the mandrel life. The mandrel may sometimes break during rolling to cause a serious trouble.